This article is part of the OWASP Testing Guide v3. The entire OWASP Testing Guide v3 can be downloaded here.

OWASP at the moment is working at the OWASP Testing Guide v4: you can browse the Guide here

4.10 Web Services Testing

SOA (Service Orientated Architecture)/Web services applications are up-and-coming systems which are enabling businesses to interoperate and are growing at an unprecedented rate.
Webservice "clients" are generally not user web front-ends but other backend servers.
Webservices are exposed to the net like any other service but can be used on HTTP, FTP, SMTP, MQ among other transport protocols.
The Web Services Framework utilizes HTTP protocol (as standard Web Application) in conjunction with XML, SOAP, WSDL and UDDI technologies:

The "Web Services Description Language" (WSDL) is used to describe the interfaces of a service.

The "Simple Object Access Protocol" (SOAP) provides the means for communication between Web Services and Client Applications with XML and HTTP.

"Universal Description, Discovery and Integration" (UDDI) is used to register and publish Web Services and their characteristics so that they can be found from potential clients.

The vulnerabilities in web services are similar to other vulnerabilities, such as SQL injection, information disclosure and leakage, but web services also have unique XML/parser related vulnerabilities, which are discussed here as well.